For detecting flaws (including cracks) existing on a steel member, there is a known technique wherein the surface region of the member is heated by high frequency induction heating, and the flaws on the surface of the member are detected by measuring the surface temperature of the member with a radiation thermometer. This technique is disclosed in Japanese Provisional Publication No. 298846/90 and in corresponding U.S. Pat. No. 5,069,005. As shown more specifically in FIG. 36, the surface region of the member W to be inspected is heated by using a high frequency induction heating coil 8 during transportation, and then the temperature distribution on the surface of the member W is measured with a radiation thermometer 9 arranged just next to the coil 8. If a flaw exists on the surface, the temperature at the flaw is different from the sound part (higher or lower than the surroundings, for example) so that the flaw is detected according to this temperature difference.
Although a member covered with scale can be adopted for the flaw detection mentioned above, the member is treated by shot blasting as a pretreatment for removing the scale prior to the flaw detection. The surface of the member W becomes a glossy one (shot surface) after the shot blasting and reduces the emissivity .epsilon. so that the temperature difference .DELTA.T detected by the radiation thermometer 9, i.e., the level of the flaw signal decreases. Furthermore, handling marks (for example, marks due to roller rubbing, bar applying or wire applying) are easily formed on the surface of the member, and these marks exhibit mirror-like appearance which is much more glossy than the shot surface so that the emissivity .epsilon. decreases significantly thereat. The part of lower emissivity .epsilon. is detected as a lower temperature part with the radiation thermometer 9. Therefore, it may be difficult to judge whether the low temperature part detected by the radiation thermometer 9 shows a real flaw or a handling mark. This leads to a false detection of regarding a handling mark as a real flaw and may lead to decrease in the detection accuracy.
The U.S. Pat. No. 3,020,745 discloses a technique wherein the surface of the member is uniformly covered with paint containing a black-body powder, such as carbon or graphite powder, for enhancing the emissivity .epsilon. of the member according to the heat radiation from the powder. This technique seems effective for achieving a high accuracy of flaw detection. Furthermore, the U.S. Pat. No. 3,504,524 teaches a non-destructive type of similar method wherein the painted layer is peeled off after flaw detection.
Although this is not a method for flaw-detection, the U.S. Pat. No. 4,408,903 discloses a method for measuring temperature of a member in hot-working process, such as hot-rolling or hot-forging, wherein carbon powder is electrodeposited on a surface of a long member in continuous transportation, whereby the heat radiation from the surface is homogenized, and high accuracy of temperature measurement is achieved. The member is generally heated by using an induction heating coil in such type of temperature measuring system. It is important to measure the member temperature just after heating in order to secure a precise temperature controlling condition in a hot-working process. For accomplishing such object, the U.S. Pat. No. 4,317,978 discloses an induction heating coil wherein the coil has a window for temperature measurement by a radiation thermometer.
Incidentally, when the flaw detection is performed on a member covered with scale, the handling marks by rolls, bars, wires, and so on, mentioned above may appear as parts of scale peeling off. The emissivity is smaller also for such parts than for the surroundings (scale), and the temperature measured with the radiation thermometer becomes lower thereat. This situation is fundamentally the same as that of the handling marks formed on the shot surface.
In this case, it appears that the thickness of the powder layer should be adjusted carefully for preventing such problem. In the prior arts, however, there is no disclosure or suggestion about the preferable powder layer thickness for preventing such false detection. A power layer on the member surface may prevent the heat from reflecting at the handling marks, which may lead to a false flaw detection. Too much thickness of coverage may, however, cause a decrease in heat radiation from the powder layer because a very thick powder layer is hardly heated to a sufficient temperature only by a heat conduction from the member. This may cause a decrease in the flaw-detection sensitivity due to a lower level of the flaw signal.
In this point of view, the powder layer seems to be preferably formed as thin as possible, as suggested in the U.S. Pat. No. 3,020,745. However, there is no disclosure on the preferable range of the thickness. For preventing the heat reflection at the handling mark, it seems to make a sense for a person having ordinary skill in the art that too thin powder coverage may increase exposing surface area without coverage on the member surface, whereby the effect in increasing the flaw signal due to the heat radiation from the powder layer cannot be expected sufficiently, and the probability of the false detection may increase because of a non-uniform powder coverage allowing partial exposure of handling marks. Therefore, the preferable configuration of the powder layer seems to be thin and uniform one, i.e., that without causing any surface exposure on the member, as suggested in the U.S. Pat. No. 3,020,745.
Furthermore, the important factor for achieving such uniform powder layer without causing exposure is not only the thickness thereof but also the particle diameter of the powder to be used. As shown in FIG. 35 (a), when the average thickness of the powder layer is fixed to t0, the layer consisting of particles with small diameter may hardly causes surface exposure since the particle layers may be multiply formed therein. On the other hand, as shown in FIG. 35 (b), the number of the particle layers decrease with increasing the diameter of the particle, and the member surface may expose at the interval between the particles. The U.S. Pat. No. 3,020,745, however, discloses no information about the preferable range of the powder layer thickness in relation to the powder diameter. Also in this case, it seems to make sense for a person having ordinary skill in the art that multilayered powder coverage as shown in FIG. 35 (a) is more preferable for preventing the exposure of the member surface.
The next problem is a configuration of the induction heating coil. Although the flaw detecting process has a technical similarity with the temperature measurement in the hot-working process in respect of measuring member temperature with a radiation thermometer, the technical purposes are completely different from each other. In the hot-working process, the member should be heated uniformly to high temperature so that the member is sufficiently softened for easy deformation. Therefore, a long coil is used for this purpose for securing a sufficient heating time as disclosed in the U.S. Pat. No. 4,317,978 wherein the coil length is almost ten times as long as the diameter of the coil cavity. In the case of flaw detection, however, such long coil causes an extreme decrease in flaw detection accuracy since the temperature difference between a flaw part and a sound part is completely vanished due to a long and uniform heating to high temperature.
The object of this invention is to enable a high accuracy of flaw detection even in the case that the surface emissivity of the member is not uniform because of the handling marks mentioned above, particularly by specifying the configuration or the morphology of the powder layer. Furthermore, it is also the object of this invention to make the flaw detection more easy and reliable by enhancing the total surface emissivity of the member and increasing the temperature difference .DELTA.T (i.e., the flaw signal) on the surface.